Constants saved for reference

They still need refer to synthetic units like the metre, joule and arguably so the second; and those were established long before the ISO found nuclear-physics level ways to describe them.

The physical metre standard is still a length of platinum bar kept in France, so the ISO mathematicians had somehow to define it to limits far beyond any normal measuring tools.

So I wonder how they satisfy the circular argument. You can define the metre as indeed they do, by so-many wavelengths of light of a certain frequency, but to do that means choosing a calibration light that can be used as such. I think it's the emission light of Sodium but there seems no particular reason for that choice.

Essentially it's making the formal, physicists' definition of the metre, still that of the length of a carefully-preserved metal bar made to be a certain fraction of a geographical distance measured as accurately as 18C surveyors could manage!

Then of course you need be sure the measuring system is calibrated properly...

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Don't get me wrong: I am not knocking the Metric System, nor the Systéme Internationale although some of the latter's units are clumsy for practical use.

Modern science and engineering, and commerce, would really struggle without their consistency and arithmetical simplicity despite some practical cost such as the Radian and Pascal (though it lets us use the Degree and Bar for "ordinary" work).

Hence its adoption worldwide, apart from everyday use in America and a few specific exceptions in the UK, but all technical work in these two nations uses the ISO units.


Indeed, France invented the Metric System for simplicity, as its own home-grown weights and measures had become an utter tangle of individual units by trade and region; severely hampering trade even within the country. Since France trades with several countries with direct frontiers it was not long before the neighbours copied it, and so the Metric System gadually and patchily spread world-wide.

The UK was quite slow to adopt it: we Britons love individuality! The roads and railways still use the Statute Mile and Yard (and the Chain, on the railways); ales and ciders are still dispensed by the pint in pubs. Some engineering and building materials are still made to inch dimensions. Otherwise it's all Metric / ISO although schools wrongly insist on teaching the "non-preferred" centimetre.

There is a story that the fledgling USA - which still uses a 16C vintners' version of the Gallon - might have adopted the Metric System from the start had a ship carrying representative Standards not sunk!

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I have found one unexpected exception. In at least some European countries, organ-pipe lengths are still specified in feet (e.g. Diapason 8', Tibia 4'). Well of course: Bach, Saint-Saens or Lapwood would just not sound the same in Diapason 2438.4.... Let alone Na-emission wavelengths.

The base units are now defined in terms of the constants.